To facilitate studies of the effects of expression of HIV envelope proteins, we made an expression vector by deleting segments of gag and pol genes from the infectious HIV provirus clone pNL4-3. Following transfection of this into a variety of human cell lines, HIV envelope proteins are made in amounts comparable to those made after transfection of wild-type HIV proviral DNA. A syncytia assay was developed which detects HIV envelope proteins following transfection of HIV envelope-expressing plasmid DNAs into a modified HeLA cell line which expresses on its surface the HIV receptor molecular CD4. Using this assay, we found that mutations in two portions of the HIV env gene ablate syncytia formation: a region of the env gene which is necessary for binding to CD4 , and a hydrophobic region at the amino terminus of the gp41 env peptide. We plan to use these envelope mutants to study the effects of long term HIV envelope expression in CD4 positive cells, with particular interest in how the HIV envelope affects expression of CD4 and infectibility of HIV.